The invention is concerned with a process for catalyzing cationic paint binders. More particularly, the invention relates to catalyzed paint binders of the type used in cathodic electrodeposition coatings.
Cathodically depositable electrodeposition paints essentially are crosslinked at elevated temperature through transesterification, transamidation, transurethanization, or through the reaction of chain end double bonds. It is known that crosslinking reactions of this type are catalyzed with metal compounds. In practically all cases, catalysis for the curing of cathodically depositable paint films is necessary to achieve the performance required for commercial applications. One of the more important catalysts used in practice are compounds of lead II, used either as lead(II) oxide or in the form of its salts with longer chain fatty acids (naphthenic acid, octanoic acid), as well as with acids forming water-soluble salts (formic acid, acetic acid, lactic acid, etc.) or as a pigment (basic lead silicate) forming, partially at least, a water-soluble salt through reaction with the acid neutralizing the cathodic binder. All catalysts presently used for this purpose, however, lead to essential disadvantages in cathodically deposited electrodeposition paints.
The lead compounds which are solid substances (lead oxide or basic lead silicate) dissolve only in the paint. Thus, the paints have to be homogenized for substantial periods to achieve the effectivity of the lead salt as a catalyst. The water-soluble salts which form--as well as other water-soluble salts directly added to the paint--in their essential part are not in the resin micelle, but rather are in the aqueous phase of the paint. On electrodeposition they are washed out by the electrical endosmosis in the film causing dewatering of the film, or they are deposited on the substrate as metal precipitate. Furthermore, portions of the dissolved salts are removed from the paint bath due to the dialysis installations in electrodeposition plants and, where a rinsing process is used, are carried back into the tank without adequate control. The water-insoluble salts of longer chain fatty acids are soluble in the resin and, thus, substantially remain in the resin micelle. Through hydrolysis, however, titratable quantities of water-insoluble fatty acids form, which essentially disturb the deposition behavior and the control of the paint bath on electrodeposition.
According to EP-A1 No. 00 82 214, attempts were made to fix the catalytically effective metals by reaction with a tertiary amino group and hydroxy group containing binder. However, such reactions require temperatures of from about 100.degree. C. to 150.degree. C. which will bring about crosslinking of the reactive resin types presently preferred. Furthermore, for the procedure suggested, the resin must contain a relatively high level of organic solvents. This complicates the formulation of paint baths with low-solvent content, i.e., low-VOC or low volatile organic composition types, to a substantial extent.